The determination of the electronic structure of molecules can be an important element in an understanding of their biological function. It is the general purpose of the research proposed here to use certain chemical techniques for the elucidation of the molecular electronic structure where the available information suggests that electronic changes (such as those involved in excitation, charge transfer, conformational alterations, etc.) may be essential for the biological activity. We propose that for the appropriate systems, experimental spectroscopy combined with quantum mechanical calculations can provide a knowledge of the essential aspects of electronic structure. This knowledge may make possible further progress in obtaining a detailed interpretation of the biological function. Clearly, such a proposal can be realized only for suitable problems. One obvious case is provided by the visual pigments to which we shall devote most of our time for the duration of this grant. BIBLIOGRAPHIC REFERENCES: A. Szabo and M. Karplus, Analysis of Cooperativity in Hemoglobin. Valency Hybrids, Oxidations, and Methemoglobin Replacement Reactions, Biochemistry 14, 931 (1975). B. Honig, A. Warshel, and M. Karplus, Theoretical Studies of the Visual Chromophore, Accts. Chem. Res. 8, 92 (1975).